Calendar Color is a colour picker (with a palette of preset colours) on the venue form. On the Calendar (monthly, weekly and daily views) every proposal held at that venue, and every online venue booking, is drawn with that background colour and an automatically contrasting text colour, so you can tell your venues apart at a glance. Leave it empty to use the default white card with just the status dot. The Calendar itself requires both the Proposals and Venues modules to be enabled.